Various power units intended to be used as pressure-fluid operated arrangements are widely known, such as pressure-fluid operated motors and pumps. There are various kinds of pressure-fluid operated motors, such as piston motors, screw motors, gear motors and vane motors. Different kinds of pressure fluid pumps are also known, e.g. piston pumps, screw pumps, gear pumps and vane pumps. The same power unit often functions both as the motor and the pump, whereby e.g. a hydraulic pump and a hydraulic motor connected to it may be identical in the same device.
Both the structure and production of power units comprising pistons are usually complicated, and thus they have gaskets and several parts prone to wear. It is rather expensive to produce them, and their service increases the operating costs considerably. In screw-type arrangements, on the other hand, the screw mechanism is expensive and difficult to manufacture. Vane motors and gear motors as well as vane pumps and gear pumps are relatively cheap to produce, but the efficiency of vane-type power units is poor in all respects, and their operation is very inaccurate.
A further problem of the prior art solutions is that when they are used for operating an actuator, it is rather difficult to guide the actuator to a predetermined position accurately, and controlling of a certain alternating working motion, for example, requires additional control systems, which makes the use of the conventional constructions difficult and expensive.